Measurements of residual enzyme activity and FTIR-spectroscopy revealed that mushroom polyphenoloxidase is a thermosensitive enzyme readily inactivated by temperatures exceeding 50°C. The enzyme is, however, very pressure stable. At room temperature no enzyme inactivation occurred at pressures up to 6 kbar. FTIR-spectroscopy revealed that a modification in enzyme conformation occurred below 9 kbar. Kinetic studies showed that pressure and temperature did not always act synergistically with respect to enzyme inactivation. FTIR-spectroscopy showed that pressure- or temperature-induced changes were considerably different. Both temperature and pressure-temperature inactivation were only slightly influenced by pH (in the pH-range 5.5–7.5).